I would buy some BTC, ETH and also 3rd generation of cryptos like ADA and IOTA.

There are also smaler coins where you can start without money and by participating in bounty programs you can actually earn a lot. I earned 675 VIBs just from yesterday, that is the Viberate token. And I spent about 20 minutes doing this. In 2 weeks I got about 4k VIB. That is about 350 euros right now.

Check it out: https://www.viberate.com/referrals/2ed6b8b9_9588dc25_2edd6fe5

If i had $50 for invest, I probably invest in waves. Actually I did it last week and ı am gonna collect waves as much as ı can. Becasuse I believe it's future will be great. And I don't wanna be sad later by do not invest in this coin. I am sure that I won't be regret.

if you only have $ 50 then you need to find a coin for the price of 1 satosh.
 it is not easy to do this, you will lose a lot on the transfer from the exchange to exchange, but there is a reason to make good money

there's nothing wrong if you want to try your luck with the $50 your own. 
to experience this is enough, and if it fails, it should no matter, right. so, go on. do your best. Maybe you can start with a token that has a red value.

I would like to use it to buy coins with higher potential, but it will surely take a long time or I will use it in risk, atleast I have chance to earn more and buy more coins if ever I got some profit. but I'm always on the safe side, I will surely exchange it to fiat if I only have $50  

You can invest in waves.
at least 50 dollars would be 300 dollars it does not make sense to get a high priced coin.

If I had only 50$,I would have invested in NXT, Bitshares and Ardor.

Actually worked out quite well, and Waves is just getting started. Starting to gain momentum to turn into a Tsunami in 2018.

to be honest $50 doesn't sound adequate if you want to invest, knowing the fact that it will take much more money to at least gain significant benefit from your investments. However, if you are willing to take the risk, I would say just go for the hot upcoming ETH token's projects that are likely to skyrocket upon its ICO announcement. It takes time and sufficient knowledge to spot these types of projects, but when you do it is going to be a great source of income.